1. Field of the Invention
The present invention relates to a brush holder device for use in a small-size motor used for driving, for example, a power tool.
2. Description of the Related Art
The brush holder device of the present invention can be used in ordinary small-size motors. An example of such a small-size motor is described below with reference to FIG. 5.
FIG. 5A is a half sectional view showing a small-size motor of conventional configuration; FIG. 5B is a half sectional view showing an end bell of synthetic resin when removed from a housing of the motor of FIG. 5A; and FIG. 5C is a half sectional view showing an end cover for the housing of the motor of FIG. 5A. A housing 9 is formed from a metallic material into a closed-end hollow cylindrical shape and has a magnet 12 mounted on the inner wall thereof. An end cover 15 is fitted to an end opening portion of the housing 9 to thereby close the housing 9. A bearing 16 for supporting one end of a shaft 17 is housed in a central portion of the end cover 15. An opposite end of the shaft 17 is supported by a bearing 13 provided at the center of a closed-end portion of the housing 9.
The shaft 17 carries a laminated core 18, windings 19 wound onto the laminated core 18, and a commutator 10, thus forming a rotor of the small-size motor. Brushes in contact with the commutator 10 are connected to brush arms. Input terminals 11 connected to the brush arms extend through the end cover 15 and project outward from the end cover 15 for electric connection.
As described above, a brush holder device of the small-size motor is composed of a brush, and a brush arm for supporting the brush. The brush arm is mechanically and electrically connected to an input terminal supported by an end bell 14. Such a conventional brush holding technique will next be described with reference to FIGS. 3 and 4.
FIG. 3 shows a state of a brush being mounted on a brush arm according to a conventional technique, wherein FIG. 3A is a view of the brush as viewed from a side opposite a side for contact with a commutator, and FIG. 3B is a side view. FIG. 4 shows components of the configuration of FIG. 3 in an exploded condition, wherein FIG. 4A shows a tip portion of the brush arm, and FIG. 4B shows a brush.
An end of a brush 7 that is opposite an end for contact with a commutator is attached to a brush arm 1. In order to enhance longitudinal rigidity, the brush arm 1 is bent along laterally opposite edges (see FIG. 4A). In order to allow attachment of the brush 7 to its tip portion, the brush arm 1 has an engagement hole formed in the tip portion such that the engagement hole assumes the same shape as that of an engagement portion of the brush 7 so as to receive the brush 7 and such that laterally opposite edge portions of the engagement hole are bent to form brush contact portions 4. Notably, a portion of the brush arm 1 other than the tip portion is not illustrated. When the brush 7 is fitted into the engagement hole of the brush arm 1, the brush contact portions 4 support the brush 7 while maintaining mechanical and electrical contact with the brush 7.
As shown in FIG. 3, the brush 7 attached to the brush arm 1 undergoes bonding by use of an electrically conductive adhesive 8; for example, an epoxy adhesive mixed with silver. Thus is completed assembly of the brush holder device.
However, the brush holder device involves high cost in assembly, since the electrically conductive adhesive 8 is expensive. Additionally, use of the electrically conductive adhesive 8 impairs work efficiency, since, for example, the adhesive 8 requires drying. Particularly, in view of the brush arm 1 serving not only as a current path to the brush 7 but also as a heat radiator for releasing heat generated from sliding contact between the brush 7 and a commutator, a thicker brush arm 1 is advantageous. However, the thickness of the brush arm 1 is limited, since the brush arm 1 must cause the brush 7 to be in contact with the rotating commutator at constant brush tension while absorbing vibration induced by the brush 7 being in contact with the surface of the commutator, which is not smooth due to gaps between commutator segments. As a result, the conventional brush arm 1 fails to exhibit sufficient heat radiation effect.